fix(cli): reject empty messages without attachments

[rmarquesAvatel]

Summary

• Reject buzz messages send when trimmed content is empty and no --file is present.
• Return the existing usage-error shape and exit code 1 before relay lookup, upload, or publication.
• Preserve text-only and attachment-only sends with relay-free process coverage.

Work package: BUZZ_PAQUETE_RECHAZO_CUERPO_VACIO_2026_08_30.md (internal factory package; audit ratification event 4d70c1beed11218e46c4fe2ed5326461ea1db2ae947fd2b2de01b4a85e0ba59c).

Evidence

• Red on base eed74bde2f4797714335ac10c56c0b0244c1def4: focused test had 3 expected rejection failures and 2 pass-through passes.
• Green on candidate 27b763353e7ae688733282ff8dbe62c9e521a692: focused test 5/5; just ci passed.
• Real relay: empty send exited 1 with zero author events since the fixed cursor; attachment-only and normal text sends were published and read back (full IDs and filters retained in the delivery notes).

Test plan

• cargo test -p buzz-cli --test messages_send_empty
• just ci

[github-actions]

🔐 Codex Security Review

Status: review required for the current range.

The current range is eed74bde2f4797714335ac10c56c0b0244c1def4...27b763353e7ae688733282ff8dbe62c9e521a692.
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@buzz-security-review 27b763353e7ae688733282ff8dbe62c9e521a692 to authorize a new review.
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